Recyclable ligand-free mesoporous heterogeneous Pd catalysts for Heck coupling

Article abstract of DOI:10.1016/j.tetlet.2005.09.048

Ligand-free Pd-MCM41 catalysts are highly active in the Heck coupling of bromoarenes including deactivated bromo derivatives to give coupled products in high yields with high selectivities without the need to exclude air or moisture. The catalyst samples exhibit unprecedented stability among heterogeneous catalysts and can be reused at least 20 times to achieve complete conversion without any additional activation treatment.

Full text of DOI:10.1016/j.tetlet.2005.09.048

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 7725–7728
Recyclable ligand-free mesoporous heterogeneous Pd catalysts
for Heck coupling
a
b
a,
*
´
´
´
´
´
Attila Papp, Gabor Galbacs and Arpad Molnar
aDepartment of Organic Chemistry, University of Szeged, Do´m te´r 8, Szeged H-6720, Hungary
bDepartment of Inorganic and Analytical Chemistry, University of Szeged, Do´m te´r 7, Szeged H-6720, Hungary
Received 4 August 2005; revised 2 September 2005; accepted 7 September 2005
Available online 22 September 2005
Abstract—Ligand-free Pd–MCM41 catalysts are highly active in the Heck coupling of bromoarenes including deactivated bromo
derivatives to give coupled products in high yields with high selectivities without the need to exclude air or moisture. The catalyst
samples exhibit unprecedented stability among heterogeneous catalysts and can be reused at least 20 times to achieve complete
conversion without any additional activation treatment.
ꢀ 2005 Elsevier Ltd. All rights reserved.
The Heck reaction1 is an established, useful protocol for
carbon–carbon bond formation and has found wide-
spread applications in the stereoselective synthesis of
various alkenes. Usually aryl halides are reacted with
alkenes in the presence of palladium and a suitable base
in a single step under mild conditions.2 Efforts have
recently been made to develop new catalysts to replace
air- and moisture-sensitive phosphine ligands, which
also allow the use other aryl derivatives. These new cat-
alysts include palladacycles,3 N-heterocyclic carbene–
palladium catalysts,4 a macrocyclic recoverable triolefin
complex,5 and thiourea–Pd(0) complexes.6 Attention
has also been paid to the search for new, efficient, and
recyclable heterogeneous catalysts. Among various
strategies, incorporation/grafting of metal complexes
into suitable solids such as polymers7 or porous siliceous
materials8 have been demonstrated to be promising ap-
proaches. Equally useful catalysts can be prepared by
entrapping9 or immobilizing10 metal nanoparticles, and
generating and depositing active metal particles onto
solid supports.11 In this context, the recent observation
that Pd(OAc)2 is a highly active catalyst in the Heck
reaction in low concentrations (catalyst amount below
0.1 mol %) is a significant new development,12 but fur-
ther improvement is needed. Decomposition of the cat-
alytically active component, metal leaching, insufficient
catalyst activity, and difficulties with respect to recovery
and recycling properties are the major problems requir-
ing significant effort.
Heterogeneous systems may offer solutions for some of
these problems. Ordered mesoporous silica materials
prepared by micelle-templated synthesis13 have recently
found wide application as catalyst materials. Among
various possibilities, surface modification has been used
quite extensively to produce active catalytic materials.
The functional groups of such hybrid organic–inorganic
materials14 may serve as anchoring sites for metal com-
plexes (heterogenization of homogeneous catalysts)13b,f
or, having suitable catalytic properties, they may act
as surface active sites.13f,15 Mesoporous silicas may also
serve as supports for metal particles.13a,b,f,15c
Metallic palladium has been incorporated into various
silica materials including amorphous silica11e,f,16 and
ordered mesoporous silicas such as MCM-41,11a,16,17
HMS,11c SAB-15,11c and ETS-10.11e These heteroge-
neous systems have been used as catalysts in hydrogena-
tion11e,16,17 and in various coupling reactions.11e,f In
fact, Pd–MCM-41 prepared by vacuum deposition has
already been applied in the Heck coupling.11a
We have found that a new, heterogeneous ligand-free
Pd–MCM-41 system is an efficient and recyclable cata-
lyst in the Heck coupling. For our catalyst synthesis
we used a literature method18 described for the synthesis
Keywords: C–C Coupling; Heck reaction; Palladium; MCM-41;
Heterogeneous Catalysis; Recycling.
*
Corresponding author. Tel.: +36 62 544 277; fax: +36 62 544 200;
e-mail: amolnar@chem.u-szeged.hu
0040-4039/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.09.048

7726
A. Papp et al. / Tetrahedron Letters 46 (2005) 7725–7728
of MCM-41 and modified it for palladium incorpora-
tion.19 Catalysts were generated via simultaneous self
assembling of mesoporous MCM-41 silica and particle
generation. Three catalysts with palladium loadings of
1.39%, 3.0%, and 5.85% based on inductively coupled
plasma atomic emission spectroscopy (ICP-AES) were
made and characterized by various physical methods.
BET surface areas of the samples calculated by the
BJH method were found to decrease (1099, 979, and
806 m2/g, respectively), with increasing palladium load-
ing, whereas pore diameters and d100 indexed unit-cell
sizes determined by low-angle XRD increase slightly.
The 1.39% Pd–MCM-41 sample exhibited three reflec-
1
R
X
1
R
Pd-MCM41
NMP, base, 150 o
2
+
R
C
2
R
yields = 54–100%
selectivites = 71–100%
X = I, Br
1
R
R
= H, Cl, Br, OMe, Ac, NO ,
2
C
N
2
= Ph, COOMe
Scheme 1. The Heck reaction.
Table 2. Pd-catalyzed Heck reaction of bromoarenes over 1.39% Pd–
MCM-41 catalysta
Aryl halide
Alkene
Time Conv. Sel.b
tions characteristic of the MCM-41 structure (d100
—
(h)
(%)
(%)
2.35ꢁ, d110—4.07ꢁ, d200—4.6ꢁ) but increasing Pd loading
resulted in the formation of less ordered structures. Pd
dispersion values measured by hydrogen chemisorption
were 8.2%, 5.7%, and 13.6%, respectively. Further char-
acterization data can be found in a recent publication.20
Bromobenzene
Me acrylate
Styrene
3
6
86
93
96
93
4-Bromobenzonitrile
4-Bromoanisole
1,4-Dibromobenzene
Me acrylate
Styrene
3
6
97
90
93
95
Me acrylate
Styrene
3
6
82
91
94
89
All three catalysts showed high activity and selectivity in
the Heck coupling of iodobenzene with styrene (conver-
sions: 76–84%, selectivities: 84–87%) and methyl acryl-
ate (89–100% conversions and 100% selectivities)
(Table 1).21 Moreover, activities in the transformation
of activated bromoarenes were also satisfactory. In all
reactions complete E selectivity was observed, whereas
in the transformations of styrene, products of a coupling
were found (Table 1 and Scheme 1). It was also seen that
activities depend on palladium loadings and parallel
metal dispersion: in most cases catalytic performance
of 3.0% Pd–MCM-41 with the lowest Pd dispersion
(5.7%) showed lower activity.
Me acrylate
Styrene
3
6
100
85
93
92
1-Bromo-4-chlorobenzene Me acrylate
Styrene
3
6
64
54
100
91
1-Bromonaphthalenec
Me acrylate
Styrene
3
6
100
100
100
92
3-Bromopyridined
Me acrylate 22
Styrene 22
94
57
100
93
a Reaction conditions: 1 equiv of aryl halide, 1.2 equiv of alkene, and
Na2CO3, 0.2 equiv of Bu4NCl, 0.3 mol % Pd.
b In all reactions complete E selectivity was observed. Side products:
transformation of styrene—the product of a coupling, transforma-
tion of methyl acrylate–butyl cinnamate.
In further studies a wide range of other aryl halides were
reacted with styrene and methyl acrylate and appropri-
ately selected reaction conditions, namely the use of var-
ious bases and additives and the ratio of the reacting
components, were tested. Aromatic chloro compounds
were found not to react but high reactivities could be
achieved with deactivated bromoarenes (4-bromoani-
sole, 1-bromo-4-chlorobenzene, 1,4-dibromobenzene)
(Table 2). Under these conditions (the use of a small
excess of alkene and Na2CO3 with Bu4NCl as additive),
c Without Bu4NCl.
d Without Bu4NCl, with NaOAc as the base.
bromobenzene also showed increased reactivity. The
use of Bu4NCl as additive was particularly beneficial
in harmony with earlier observations.2b,22 Interestingly,
however, both 1-bromo-naphthalene and 3-bromopyri-
dine gave much better results without the additive.
Table 1. Pd-catalyzed Heck reaction over Pd–MCM-41 catalysts with various Pd loadingsa
Aryl halide
Alkene
1.39%
Conversionb
3.0%
Conversionb
5.85%
Conversionb
Selectivityc
(%)
Selectivityc
(%)
Selectivityc
(%)
(%)
(%)
(%)
Iodobenzene
Styrene
Me acrylate
84
100
84
100
76
98
86
100
82
89
87
100
4-Iodoanisole
Styrene
93
80
83
80
88
80
Me acrylate
100
100
100
100
100
100
4-Bromonitrobenzene
4-Bromoacetophenone
Styrene
Me acrylate
97
100
95
100
100
99
95
100
95
100
95
100
Styrene
Me acrylate
97
90
93
100
77
91
93
100
89
84
100
92
a Reaction conditions: equimolar amounts of aryl halide, alkene, and Na2CO3, NMP solvent; time: 1 h for iodobenzene, 2 h for all other aryl halides.
b Determined by GC.
c In all reactions complete E selectivity was observed. In the transformation of styrene, the product of a coupling was also formed.

A. Papp et al. / Tetrahedron Letters 46 (2005) 7725–7728
7727
Table 3. Recycling of Pd–MCM-41 catalysts and 10% Pd-on-carbon in the Heck coupling of iodobenzene with methyl acrylatea
Entry
Catalyst
Run 1
Run 3
Run 5
Run 7
Run 8
Run 10
Run 12
1
2
3
4
5
6
1.39% Pd–MCM-41
1.39% Pd–MCM-41b
1.39% Pd–MCM-41c
3.0% Pd–MCM-41
5.85% Pd–MCM-41
10% Pd-on-carbon
85
85
93
76
80
87
75
97
93
77
75
72
83
98
93
77
82
35
53
86
80
77
79
0
19
34
65
54
82
0
0
44
1
0
0
28
0
60
20
a Reaction conditions: equimolar amounts of aryl halide, alkene, and NEt3, 150 ꢁC, 60 min.
b After the first use 10 lL of 4 · 10À2 M iodine solution in NMP was added.
c 170 ꢁC, 45 min.
Recovery and catalyst reuse are important issues in the
Heck coupling. Easy catalyst separation and recycling
in successive batch operations can greatly increase
the efficiency of the reaction. Reports exist, which
show some success in this respect using immobilized
complexes7f,8a–d,g,23 and heterogeneous catalysts with
metallic Pd9,10,11b,f in a limited number of reuses. We
showed in an earlier study11h that new heterogeneous
Pd catalysts with organic–inorganic hybrid supports
exhibit promising characteristics in recycling studies.
Therefore, detailed studies were performed with these
new Pd–MCM-41 catalysts to explore their performance
in reuse experiments.
loss of activity. Similar excellent results were observed
for both catalyst samples using a mixture of organic
and inorganic bases (entries 2 and 3). Furthermore, both
the activity and the number of reuses could be main-
tained at lower temperature with the mixed bases (entry
4).
In conclusion, we have prepared new heterogeneous pal-
ladium catalysts supported on mesoporous MCM-41
and successfully used them in the Heck coupling of iodo-
and bromoarenes. More importantly, the catalysts are
stable under the reaction conditions and retain high
activity and selectivity for at least to 20 successive runs
without the need to exclude air or moisture. This excel-
lent catalyst performance and the easy preparation and
separation of the catalysts make these new Pd-on-
MCM-41 systems a useful alternative to other hetero-
geneous Pd catalysts.
First, all three catalysts were tested and their perfor-
mance was compared to a commercial sample. Commer-
cial 10% Pd-on-carbon rapidly loses activity (Table 3,
entry 6) whereas all Pd–MCM-41 samples maintain
activities for much longer (entries 1, 4, and 5). Treat-
ment of 1.39% Pd–MCM-41 after the first run with
iodine as described in an earlier study24 proved to be
useful to increase catalyst activity but was not suitable
to prolong the lifetime of the catalyst (entry 2).
Increased reaction temperature, however, did result in
improvements in both activities and the number of
reuses (entry 3).
Acknowledgment
We thank the Hungarian National Science Foundation
(OTKA grants T042603, TS044690, M041451, and
F043213) for financial support.
In further studies changes in the reactant ratio and a
combination of various bases were tested with 1.39%
and 5.85% Pd–MCM-41 catalysts, which led to better
performance. We observed further significant improve-
ments at increased reaction temperature upon increasing
the amount of both methyl acrylate and base by 50%
(Table 4, entry 1): complete conversion was achieved
and the catalyst could be reused 20 times without any
Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.tetlet.
2005.09.048.
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Entry Catalyst
Run Run Run Run Run
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4
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1.39% Pd–MCM-41c 100
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c 150 ꢁC, 60 min; 1.5 equiv of methyl acrylate, 1 equiv of NEt3,
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